An ultrasonic diagnostic device is a device that transmits and receives ultrasound to and from the subject, and forms an ultrasonic image based on received signals obtained in this way. Ultrasonic images formed by the ultrasonic diagnostic device include various types of images, and an M-mode image is one of them. An M-mode image is an image representing tissue motion in the subject. In an M-mode image, for example, the vertical axis is the depth axis, and the lateral axis is the time axis, and a high brightness (or low brightness) line (contour line) extending in the time axis direction shows the state of motion of a tissue toward the depth direction. In addition, an M-mode image representing tissue motion on an observation line optionally set on a B-mode tomography image is also known.
The M-mode image is used to measure the LA/AO ratio, for example. The LA/AO ratio is an index for assessing the shape of the heart, and it is a ratio between the left atrial dimension (LA) at the end of cardiac systole and the aortic dimension (AO) at the end of cardiac diastole. In conventional measurement of the LA/AO ratio, an examiner specifies positions of both left atrial walls at the end of cardiac systole and positions of both aortic walls at the end of cardiac diastole by, for example, manually moving a cursor on an M-mode image including contour lines representing movement of positions of the aortic walls and the left atrial walls.
Because, conventionally, the examiner has to manually specify measurement points in the M-mode image, there arises a problem in that measurement is time-consuming, for example. Therefore, in order to automatically set the measure points, there has been proposed a technique of performing automatic tracing of a contour line included in an M-mode image.
For example, Patent Document 1 discloses determining one point at a position on a tissue border on an M-mode image, comparing a luminance value of that point with luminance values of five neighboring pixels located at positions shifted from that point by one pixel in the time axis direction (next time point), and setting, from among the five neighboring pixels, a pixel having the smallest difference in luminance value as a tissue border in a next iteration. By repeating this processing, a line indicating motion of the tissue border is traced. On the M-mode image, the contour line is typically a relatively smooth curve that periodically flows in the time axis direction. The technique disclosed in Patent Document 1 uses such a characteristic, and sequentially searches positions of the tissue border in the time axis direction.